I have a number of Notification Customizations that I want to make with the same content, except for a single dynamic element I'd like to include denoting which program triggered the Notify action.

That is to say, instead of having 64x2 Notification Customizations, I'd rather have 1x2 , and have something equivalent to ${sys.node.#.name} in the Notification Customization which returned the name of which of the 64x2 programs that called the Notify action.

Alternatively, I could see where a big 'or' list of variables <boolean> value in the "If" statement of a program could benefit from any Notify actions being able to distinguish which variable(s) values were logically true for the If statement that triggered the program to run the "Then" branch (somewhat like #, but for variables, rather than devices, in the "If" logic).

Historically, I have used virtual devices (fake X10 or NS-defined virtual devices) to sort of achieve this in a roundabout way. 
A set of twelve programs is triggered based on various criteria to set one of twelve virtual devices to ON, wait 5 seconds, then turn its virtual device to OFF.
Meanwhile, a single OR-based notification program listens for any of the twelve virtual devices to turn ON, and then sends a single Notification Customization which leverages ${sys.node.#.name} to say which of the twelve conditions were true.

That sounds very painful to do for a new usage case where I have 64 things which can be one of two states (hence the 64x2 nomenclature above). Looking for alternatives, or a discussion of how others would approach this.

I have used "${sys.program.#.name}" for that purpose.

But I have also found in many cases, just using the "Default" email content yields a lot of good information.

Ah, thank you. Overlooked that one. 🏆